Isolable Boron Persulfide: Activation of Elemental Sulfur with a 2-Chloro-Azaborolyl Anion

The novel boron persulfide 2 LB(η(2) -S2 ) (L=[ArNC(R)CHC(R)](-) ; Ar=2,6-Me2 C6 H3 , R=tBu) was obtained by the reaction of the 2-chloro-azaborolyl anion 1 (LBCl)K(THF) with 0.25 equiv of elemental sulfur (S8 ). Persulfide 2 is labile in solution and could be converted to the cyclic tetrasulfide LBS4 (3) and hexasulfide LBS6 (4) in the presence of sulfur at room temperature and 50 °C, respectively. Desulfination of 2 with triphenylphosphine resulted in the formation of the thioxoborane LB=S (5). Alternatively, 3 and 4 could be obtained by the reaction of 1 with an excess of sulfur. Structural analysis of 2 disclosed the relatively long S-S bond of 2.1004(8) Å due to the lone-pair repulsions of the two sulfur atoms, as disclosed by DFT calculations.